Emissions control system of a combustion engine exhaust system

ABSTRACT

An emissions control system includes a Selective Catalytic Reduction device adapted to reduce emissions, an injector adapted to inject a reductant into the device, a NOx sensor disposed downstream of the device, a controller, an iterative model, and a table. The controller is configured to perform short and long term control by confirming at least one short term criteria is met. Once confirmed, the controller calculates a normalized model error utilizing the model and a signal received from the sensor, and integrates the normalized model error. If the integrated normalized model error exceeds a threshold, the controller proceeds toward the long term control. If a long term criteria is met, a current long term factor and the integrated normalized model error is applied to the table to determine a new long term factor. The new long term factor is multiplied against an energization time of the injector.

INTRODUCTION

The present disclosure relates to exhaust systems for internalcombustion engines, and more particularly to exhaust systems usingadaptive Selective Catalytic Reduction (SCR) systems for emissioncontrol.

Exhaust gas emitted from an internal combustion engine, particularly adiesel engine, is a heterogeneous mixture that contains gaseousemissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”)and oxides of nitrogen (“NO_(x)”) as well as condensed phase materials(liquids and solids) that constitute particulate matter (“PM”). Catalystcompositions, typically disposed on catalyst supports or substrates, areprovided in an engine exhaust system as part of an after-treatmentsystem to convert certain, or all of these exhaust constituents intonon-regulated exhaust gas components.

Exhaust gas treatment systems typically include Selective CatalyticReduction (SCR) devices. An SCR device includes a substrate having anSCR catalyst disposed thereon to reduce the amount of NO_(x) in theexhaust gas. The typical exhaust treatment system also includes areductant delivery system that injects a reductant such as, for example,ammonia (NH3), urea (CO(NH2)2, and other reductants. The SCR devicemakes use of NH3 to reduce the NO_(x). For example, when the properamount of NH3 is supplied to the SCR device under the proper conditions,the NH3 reacts with the NO_(x) in the presence of the SCR catalyst toreduce the NO_(x) emissions. If the reduction reaction rate is too slow,or if there is excess ammonia in the exhaust, ammonia can slip from theSCR. On the other hand, if there is too little ammonia in the exhaust,NO_(x) conversion efficiency will be decreased.

SUMMARY

An emissions control system according to one, non-limiting, embodimentof the present disclosure treats exhaust gas of a combustion engine. Thesystem includes a Selective Catalytic Reduction (SCR) device adapted toreduce emissions, a reductant injector adapted to inject a reductantinto the SCR device, a downstream NO_(x) sensor disposed downstream ofthe SCR device, a controller, an iterative model, and a lookup table.The controller includes a processor and an electronic storage medium.The iterative model and the lookup table are stored in the electronicstorage medium. The processor is configured to perform short term andlong term adaptive control by confirming at least one short termenablement criteria is met. Once confirmed, the processor calculates anormalized chemical model error utilizing, in-part, the iterative modeland a downstream NO_(x) signal received from the downstream NO_(x)sensor. The processor then integrates the normalized chemical modelerror to produce an integrated normalized chemical model error, andconfirms that the integrated normalized chemical model error exceeds anerror threshold. The processor may then proceed toward the long termadaptive control, and confirms at least one long term adaptationenablement criteria is met. A current long term adaptive factor and theintegrated normalized chemical model error is then applied to the lookuptable to determine a new long term adaptive factor. The new long termadaptive factor is multiplied against an energization time of thereductant injector.

Additionally to the foregoing embodiment, the emissions control systemincludes an upstream NO_(x) sensor disposed upstream of the reductantinjector and the SCR device, wherein the processor is configured toreceive an upstream NO_(x) signal from the upstream NO_(x) sensor tocalculate the normalized chemical model error.

In the alternative or additionally thereto, in the foregoing embodiment,the normalized chemical model error is associated with the differencebetween a model-predicted NO_(x) level taken from the iterative model,and an actual NO_(x) level taken form the downstream NO_(x) signal.

In the alternative or additionally thereto, in the foregoing embodiment,the normalized chemical model error is normalized by magnitude.

In the alternative or additionally thereto, in the foregoing embodiment,the at least one short term enablement criteria includes at least one ofa normalized error being greater than a first threshold, a NO_(x)gradient being less than a second threshold, a reductant-consumed beinggreater than a third threshold, a temperature being greater than afourth threshold and less than a fifth threshold, a temperature gradientbeing less than a sixth threshold, a reductant storage level deviationbeing less than a seventh threshold, and a combustion mode.

In the alternative or additionally thereto, in the foregoing embodiment,the at least one long term enablement criteria includes at least one ofthe normalized error being greater than an eighth threshold, the NO_(x)gradient being less than a ninth threshold, the reductant consumed isgreater than a tenth threshold, the temperature is greater than aneleventh threshold and less than a twelfth threshold, the temperaturegradient is less than a thirteenth threshold, the reductant storagedeviation is less than a fourteenth threshold, and the combustion mode.

In the alternative or additionally thereto, in the foregoing embodiment,the at least one short term enablement criteria is independent from theat least one long term enablement criteria.

An emissions control system for treating exhaust gas of a combustionengine according to another, non-limiting, embodiment includes aSelective Catalytic Reduction (SCR) device, a first NO_(x) sensor, and acontroller. The controller is configured to perform short term and longterm adaptive control by comparing a first NO_(x) measurement from thefirst NO_(x) sensor with a predicted NO_(x) value based at least in-parton an initial chemical model. In response to a short term enablementcriteria being met, the controller calculates a normalized chemicalmodel error, integrates the normalized chemical model error, andcalculates a new long term adaptive factor if the integrated normalizedchemical model error exceeds a threshold.

Additionally to the foregoing embodiment, the emissions control systemincludes a lookup table stored in the controller and configured to crossreference a current long term adaptive factor to the integratednormalized chemical model error to calculate the new long term adaptivefactor.

In the alternative or additionally thereto, in the foregoing embodiment,the normalized chemical model error is equal to a delta between thefirst NO_(x) measurement and a predicted NO_(x) value based on theinitial chemical model, and normalized based on magnitude.

In the alternative or additionally thereto, in the foregoing embodiment,the first NO_(x) sensor is located downstream from the SCR device.

In the alternative or additionally thereto, in the foregoing embodiment,the emissions control system includes a second NO_(x) sensor, whereinthe normalized chemical model error is based on the initial chemicalmodel, the first NO_(x) measurement, and an upstream NO_(x) measurementfrom the second NO_(x) sensor located upstream from the SCR device, andwherein the first NO_(x) sensor is located downstream from the SCRdevice.

In the alternative or additionally thereto, in the foregoing embodiment,the emissions control system includes a temperature sensor configured tosend a temperature measurement to the controller, wherein the initialchemical model is generated by the controller and is at least based onthe temperature measurement, the upstream NO_(x) measurement, and thefirst NO_(x) measurement.

In the alternative or additionally thereto, in the foregoing embodiment,the enablement criteria is a short term enablement criteria.

In the alternative or additionally thereto, in the foregoing embodiment,the short term enablement criteria includes at least one of a normalizederror being greater than a first threshold, a NO_(x) gradient being lessthan a second threshold, a reductant-consumed being greater than a thirdthreshold, a temperature being greater than a fourth threshold and lessthan a fifth threshold, a temperature gradient being less than a sixththreshold, a reductant storage level deviation being less than a sevenththreshold, and a combustion mode.

In the alternative or additionally thereto, in the foregoing embodiment,the long term adaptive factor determination is conducted when a longterm adaptation enablement criteria is met.

In the alternative or additionally thereto, in the foregoing embodiment,the long term adaptation enablement criteria is independent from theshort term enablement criteria.

In the alternative or additionally thereto, in the foregoing embodiment,the long term adaptation enablement criteria includes at least one ofthe normalized error being greater than an eighth threshold, the NO_(x)gradient being less than a ninth threshold, the reductant consumed isgreater than a tenth threshold, the temperature is greater than aneleventh threshold and less than a twelfth threshold, the temperaturegradient is less than a thirteenth threshold, the reductant storagedeviation is less than a fourteenth threshold, and the combustion mode.

In the alternative or additionally thereto, in the foregoing embodiment,the eighth threshold is greater than the first threshold.

In the alternative or additionally thereto, in the foregoing embodiment,the emission control system includes a reductant injector, wherein thenew long term adaptive factor is generally multiplied by an energizationtime of the reductant injector.

The above features and advantages, and other features and advantages ofthe disclosure, are readily apparent from the following detaileddescription when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, advantages and details appear, by way of example only,in the following detailed description, the detailed descriptionreferring to the drawings in which:

FIG. 1 is a schematic of a motor vehicle including an internalcombustion engine and an exhaust system according to one or moreembodiments;

FIG. 2 is a schematic of the exhaust system including an emissionscontrol system;

FIG. 3 is a schematic of an SCR device of the emissions control;

FIG. 4 is a lookup table stored in and applied by a controller of theSCR device as part of a Long Term Adaptive (LTA) control feature; and

FIG. 5 is a flowchart of a method for adaptive SCR control and Long TermAdaptive (LTA) entry.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, its application or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features. Asused herein, the term module refers to processing circuitry that mayinclude an application specific integrated circuit (ASIC), an electroniccircuit, a processor (shared, dedicated, or group) and memory modulethat executes one or more software or firmware programs, a combinationallogic circuit, and/or other suitable components that provide thedescribed functionality.

A motor vehicle, in accordance with an aspect of an exemplaryembodiment, is indicated generally at 20 in FIG. 1. Motor vehicle 20 isshown in the form of a pickup truck. It is to be understood that motorvehicle 20 may take on various forms including automobiles, commercialtransports, marine vehicles, and the like. Motor vehicle 20 includes abody 22 having an engine compartment 24, a passenger compartment 26, anda cargo bed 28.

An Internal Combustion Engine (ICE) system 30 of the motor vehicle 20may include a combustion engine 32, an exhaust system 34, and acontroller 36. The engine compartment 24 may generally house thecombustion engine 32. Examples of combustion engines 32 may include adiesel engine, a gasoline or heptane engine, and others.

The engine 32 of the ICE system 30 may include a plurality ofreciprocating pistons attached to a crankshaft, which may be operablyattached to a driveline, such as a vehicle driveline, to power a vehicle(e.g., deliver tractive torque to the driveline). For example, the ICEsystem 30 may be any engine configuration or application, includingvarious vehicular applications (e.g., automotive, marine, and the like),as well as various non-vehicular applications (e.g., pumps, generators,and the like). While the ICE system may be described in a vehicularcontext (e.g., generating torque), other non-vehicular applications arewithin the scope of this disclosure. Therefore, when reference is madeto a vehicle, such disclosure should be interpreted as being applicableto any application of the ICE system 30.

Moreover, the ICE system 30 may generally represent any device capableof generating an exhaust gas stream generally directed and treated bythe exhaust system 34. The exhaust gas may include and chemical speciesor mixture of chemical species; in gaseous, liquid, or solid form, thatmay require treatment. In one example, the exhaust gas may generallyinclude gaseous (e.g., NO_(x), O₂), carbonaceous, and/or particulatematter species. An exhaust gas stream may contain a mixture of one ormore NO_(x) species, one or more liquid hydrocarbon species, and onemore solid particulate species (e.g., ash). It should be furtherunderstood that the embodiments disclosed herein may be applicable totreatment of effluent streams not comprising carbonaceous and/orparticulate matter species. Exhaust gas particulate matter generallyincludes carbonaceous soot, and other solid and/or liquidcarbon-containing species which are germane to combustion engine exhaustgas, or form within the exhaust system 34.

With reference to FIG. 2, the exhaust system 34 of the ICE system 30 mayinclude at least a portion of the controller 36, an exhaust gas conduit38 (i.e., exhaust manifold and pipe), and an emissions control system40. The exhaust gas conduit generally extends, and is in fluidcommunication with, the combustion engine 32 and a tail pipe 42 of theexhaust gas conduit 38 that may be located at the rear of the vehiclebody 22. The emissions control system 40 is fluidically connected to theexhaust gas conduit 38, such that exhaust gas (see arrows 44) thatpasses through the conduit 38 is treated by the emissions control system40 to reduce emissions before exiting to ambient through the tail pipe42. The emissions control system 40 also facilitates the control andmonitoring of the storage of one, or more, Nitrogen Oxides (NO_(x)),and/or treatment materials, to control the exhaust emissions produced bythe combustion engine 32.

The emissions control system 40 may include an Oxidation Catalyst (OC)device 46, a Selective Catalytic Reduction (SCR) assembly 48,particulate filter devices (not shown), and other exhaust treatmentdevices. The SCR assembly 48 may be located downstream of the OC device46 with respect to the exhaust conduit 38.

The OC device 46 of the emissions control system 40 may be one ofvarious flow-through, oxidation catalyst devices known in the art. TheOC device 46 may include a flow-through metal or ceramic monolithsubstrate 50. The substrate 50 may be packaged in a stainless steelshell or canister having an inlet and an outlet in fluid communicationwith the exhaust gas conduit 38. The substrate 50 may include anoxidation catalyst compound disposed thereon. The oxidation catalystcompound may be applied as a washcoat and may contain platinum groupmetals such as platinum (Pt), palladium (Pd), rhodium (Rh) or othersuitable oxidizing catalysts, or combination thereof. The OC device 46is useful in treating unburned gaseous and non-volatile HC and CO, thatare oxidized to form carbon dioxide and water. A washcoat layer includesa compositionally distinct layer of material disposed on the surface ofthe monolithic substrate 50 or an underlying washcoat layer. A catalystmay contain one or more washcoat layers, and each washcoat layer mayhave unique chemical catalytic functions.

The SCR assembly 48 of the emissions control system 40 may be adapted toreceive exhaust gas 44 treated by the OC device 46, and/or originatingfrom the combustion engine 32, and reduce Nitrogen Oxide (NO_(x))constituents in the exhaust gas 44. NO_(x) constituents may includeN_(y)O_(x) species, wherein y>0 and x>0. Non-limiting examples of NO_(x)may include NO, NO₂, N₂O, N₂O₂, N₂O₃, N₂O₄, and N₂O₅. More particularly,the SCR assembly 48 may convert NO_(x) to diatomic nitrogen (N₂) andwater.

The SCR assembly 48 may include at least a portion of the controller 36,an SCR device or canister 52, an upstream NO_(x) sensor 54, a downstreamNO_(x) sensor 56, at least one temperature sensor 58, at least onepressure sensor 60, a reductant injector 62, and a reductant supplysource 64. The SCR device 52 is in fluid communication with the exhaustgas conduit 44 for treatment of the exhaust gas 44. The NO_(x) sensor 54may be located upstream of the SCR device 52 and downstream of the OCdevice 46 for measuring NO_(x) constituents in the exhaust gas 44 beforethe exhaust gas enters the SCR device 52. The NO_(x) sensor 56 may belocated downstream of the SCR device 52 for measuring NO_(x)constituents in the exhaust gas 44 after the exhaust gas exits the SCRdevice 52. The temperature sensor 58 may be located upstream of the SCRdevice 52 and downstream of the reductant injector 62 for measuringexhaust gas temperature. Although the SCR device 52 is illustrateddownstream from the OC device 46, it is contemplated and understood thatthe SCR device 52 may be located upstream from the OC device 46.

The at least one pressure sensor 60 (e.g., differential pressure sensor)may be adapted to determine the pressure differential across the SCRdevice 52. Although a single differential pressure sensor 60 isillustrated, it is appreciated that a plurality of pressure sensors maybe used to determine the pressure differential of the SCR device 52. Forexample, a first pressure sensor may be disposed at an inlet of the SCRdevice 52 and a second pressure sensor may be disposed at an outlet ofthe SCR device 52. Accordingly, the difference between the pressuredetected by the second pressure sensor and the pressure detected by thefirst pressure sensor may indicate the pressure differential across theSCR device 52. It should be noted that in other examples, the sensorsmay include different, additional, or fewer sensors than the sensors 54,56, 58, 60 described.

The reductant injector 62 of the SCR assembly 48 may be generallymounted to the exhaust gas conduit 38 upstream of the SCR device 52(i.e., between the upstream NO_(x) sensor 54 and the temperature sensor58), and is configured to disperse a controlled amount of a reductant 66into the flow of exhaust gas 44. The reductant 66 is stored and suppliedto the injector 62 by the reductant supply source 64, and may be in theform of a gas, a liquid, or an aqueous solution (e.g., aqueous ureasolution). The reductant 66 may be mixed with air in the injector 62 toaid in the dispersion of the injected spray. The SCR device 52 utilizesthe reductant 66, such as ammonia (NH₃), to reduce the NO_(x).

The SCR device 52 of the SCR assembly 48 may include a substrate 68. Thesubstrate 68 may generally be a particulate filter (PF), such as adiesel particulate filter (DPF) coated with a SCR catalyst and adaptedto filter or trap carbon and other particulate matter from the exhaustgas 44. The substrate 68 generally includes an inlet and an outlet influid communication with exhaust gas conduit 38. In another example, thesubstrate may be a flow-through monolith type of substrate that maygenerally be made of ceramic. Further examples of substrates 68 mayinclude wound or packed fiber filters, open cell foams, sintered metalfibers, and others. The emissions control system 40 may also perform aregeneration process that regenerates the substrate 68 by burning-offthe particulate matter trapped in the filter substrate.

A catalyst containing washcoat disposed on the substrate 68, (i.e., aflow through catalyst or a wall flow filter) may reduce NO_(x)constituents in the exhaust gas 44. The catalyst containing washcoat maycontain a zeolite and one or more base metal components such as iron(Fe), cobalt (Co), copper (Cu), or vanadium (V), which can operateefficiently to convert NO_(x) constituents of the exhaust gas 44 in thepresence of NH₃. In one or more examples, a turbulator (i.e., mixer, notshown) may also be disposed within the exhaust conduit 38 in closeproximity to the injector 62 and/or the SCR device 52 to further assistin thorough mixing of the reductant 66 with the exhaust gas 44, and/oreven distribution throughout the SCR device 52. It is understood thatcatalyst compositions for the SCR function, and NH₃ oxidation function,may reside in discrete washcoat layers on the substrate 68 or,alternatively, the compositions for the SCR and NH₃ oxidation functionsmay reside in discrete longitudinal zones on the substrate 68.

The body of the substrate 68 may, for example, be a ceramic brick, aplate structure, or any other suitable structure such as a monolithichoneycomb structure that includes several hundred to several thousandparallel flow-through cells per square inch, although otherconfigurations are suitable. Each of the flow-through cells may bedefined by a wall surface on which the SCR catalyst composition can bewashcoated. The body of the substrate 68 may be formed from a materialcapable of withstanding the temperatures and chemical environmentassociated with the exhaust gas 44. Some specific examples of materialsthat may be used include ceramics such as extruded cordierite,α-alumina, silicon carbide, silicon nitride, zirconia, mullite,spodumene, alumina-silica-magnesia, zirconium silicate, sillimanite,petalite, or a heat and corrosion resistant metal such as titanium orstainless steel. For example, the substrate 68 may comprise anon-sulfating TiO₂ material. The body of the substrate 68 may be a PFdevice, as will be discussed below.

The SCR catalyst composition is generally a porous and high surface areamaterial which can operate efficiently to convert NO_(x) constituents inthe exhaust gas 44 in the presence of the reductant 66 (e.g., ammonia).In some embodiments the zeolite may be a β-type zeolite, a Y-typezeolite, a ZM5 zeolite, or any other crystalline zeolite structure suchas a Chabazite or a USY (ultra-stable Y-type) zeolite. The zeolite maycomprise Chabazite or SSZ. Suitable SCR catalyst compositions may havehigh thermal structural stability, particularly when used in tandem withthe substrate 68 as a particulate filter (PF) device or whenincorporated into a SCRF device, which are regenerated via hightemperature exhaust soot burning techniques.

The SCR catalyst composition may optionally further comprise one or morebase metal oxides as promoters to further decrease the SO₃ formation andto extend catalyst life. The one or more base metal oxides may includeWO₃, Al₂O₃, and MoO₃. In one embodiment, WO₃, Al₂O₃, and MoO₃ may beused in combination with V₂O₅.

The SCR catalyst (i.e., substrate 68) generally uses the reductant 66 toreduce NO_(x) species (e.g., NO and NO₂) to unregulated components. Suchcomponents include one or more of species which are not NO_(x) species,such as diatomic nitrogen (N₂), nitrogen-containing inert species, orspecies which are considered acceptable emissions. The reductant 66 maybe ammonia (NH₃), such as anhydrous ammonia or aqueous ammonia, orgenerated from a nitrogen and hydrogen rich substance such as urea(CO(NH₂)₂). Additionally or alternatively, the reductant 66 may be anycompound capable of decomposing or reacting in the presence of theexhaust gas 44 and/or heat to form ammonia. Equations (1)-(5) provideexemplary chemical reactions for NO_(x) reduction involving ammonia:

6NO+4NH₃→5N₂+6H₂O   (1)

4NO+4NH₃+O₂→4N₂+6H₂O   (2)

6NO₂+8NH₃→7N₂+12H₂O   (3)

2NO₂+4NH₃+O₂→3N₂+6H₂O   (4)

NO+NO₂+2NH₃→2N₂+3H₂O   (5)

It should be appreciated that Equations (1)-(5) are merely illustrative,and are not meant to confine the SCR device 52 to a particular NO_(x)reduction mechanism or mechanisms, nor preclude the operation of othermechanisms. The SCR device 52 may be configured to perform any one ofthe above NO_(x) reduction reactions, combinations of equations (1)through (5) NO_(x) reduction reactions, and other NO_(x) reductionreactions.

The reductant 66 may be diluted with water, where heat (e.g., from theexhaust) evaporates the water, and ammonia is supplied to the SCR device52. Non-ammonia reductants may be used as a full or partial alternativeto ammonia as desired. In embodiments where the reductant 66 includesurea, the urea reacts with the exhaust gas 44 to produce ammonia that issupplied to the SCR device 52. Reaction (6) below provides an exemplarychemical reaction of ammonia production via urea decomposition:

CO(NH₂)₂+H₂O→2NH₃+CO₂   (6)

It should be appreciated that Equation (6) is merely illustrative, andis not meant to confine the urea or other reductant 66 decomposition toa particular single mechanism, nor preclude the operation of othermechanisms.

The substrate 68 (i.e., SCR catalyst) may store the reductant 66 forinteraction with the exhaust gas 44. The SCR device 52 has a reductantcapacity, or an amount of reductant or reductant derivative it iscapable of storing. The amount of reductant 66 stored within the SCRdevice 52 relative to the SCR catalyst capacity of the substrate 68 maybe referred to as the SCR “reductant loading”, and may be indicated as apercent (%) loading (e.g., 90% reductant loading). During operation ofSCR device 52, injected reductant 66 is stored in the SCR catalyst ofthe substrate 68 and consumed during reduction reactions with undesiredNO_(x) species and must be continually replenished. Determining theprecise amount of reductant 66 to inject is critical to maintainingexhaust gas emissions at acceptable levels. Insufficient levels ofreductant 66 within the SCR device 52 may result in undesirable NO_(x)species emissions, referred as NO_(x) breakthrough, that may exit theexhaust outlet pipe 42. Excessive levels of reductant 66 injected intothe SCR device 52 may result in undesirable amounts of reductant 66passing through the SCR device 52 unreacted, or exiting the SCR device52 as an undesired reaction product, also referred to as reductant slip.Reductant slip and NO_(x) breakthrough may also occur when the SCRcatalyst of the substrate 68 is below a “light-off” temperature. SCRdosing logic may be utilized by the controller 36 to command reductantdosing.

The controller 36 may be adapted to be in electronic communication withaspects of the combustion engine 32, the reductant supply source 64, theinjector 62, the sensors 54, 56, 58, 60, and other components of the ICEsystem 30. The controller 36 may include a processor 70 (e.g.,microprocessor) and an electronic storage medium 72 that may be computerwriteable and readable. In one embodiment, the controller 36 may be anapplication specific integrated circuit (ASIC), an electronic circuit, aprocessor (shared, dedicated, or group), and an electronic memory thatexecutes one or more software or firmware programs, a combinationallogic circuit, and/or other suitable components that provide thedescribed functionality.

In operation, the processor 70 of the controller 36 may execute the SCRdosing logic stored in the storage medium 72, and may further receiveand process NO_(x) signals (see arrows 74, 76 in FIG. 2) from therespective NO_(x) sensors 54, 56 that are indicative of NO_(x) levels inthe exhaust gas 44 and proximate to the respective sensor location alongthe exhaust gas conduit 38. Similarly, the processor 70 may receive andprocess a temperature signal (see arrow 78) from the temperature sensor58, and a pressure signal (see arrow 80) from the pressure sensor 60.

A reductant injection dosing rate (e.g., grams per second) may bedetermined by the processor 70 by applying a SCR chemical model 82 andprocessing a feedback signal (see arrow 84) from the reductant supplysource 64 of the injector 62. Generally, the SCR chemical model 82,combined with the feedback signal 84 and the upstream NO_(x) signal 74,assists in generating a prediction of the amount of reductant 66 storedin the SCR device 52. The SCR chemical model 82 may further predictNO_(x) levels of the exhaust gas 44 discharged from the SCR device 52.Over time, the SCR chemical model 82 may be updatable by one or moreprocess values. For example, the SCR chemical model 82 may be updatableby a short term correction factor.

Referring to FIG. 3, the exhaust gas 44 is generally illustrated flowingthrough the SCR device 52. The controller 36 may be configured tomeasure the flow rate (F) of gas volume, and concentration (C) of thegas. For example, the emissions control system 40 determines an inputflow-rate (see arrow 86) of NO_(x) as FC_(NOx,in), where F is the volumeof the incoming gas 44, and C_(NOx,in) is the inlet concentration ofNO_(x) in the incoming exhaust gas 44. Similarly, an input flow-rate(see arrow 88) of FC_(NH3,in) is the volume of the flow-rate of NH₃(i.e., a reductant) in the incoming exhaust gas 44, C_(NH3,in) being theinlet concentration of NH₃. Further, compensating for the amount ofadsorption (see arrow 90) and amount of desorption (see arrow 92), andthe amounts reacted on the catalyst surface, the controller 36 maydetermine C_(NH3) as the SCR concentration of NH₃, and C_(NOx) as SCRconcentration of NO_(x). Accordingly, FC_(NOx) is the NO_(x) outletvolume flow rate (see arrow 94) of NO_(x) through the outlet of the SCRdevice 52. In one or more examples, the controller 36 may determineW_(NOx)FC_(NOx) as mass flow rate of NO_(x), where W_(NOx) is themolecular weight of NO_(x). Similarly, for NH₃, the outlet volume flowrate (see arrow 96) is FC_(NH3) with the mass flow rate of NH₃ beingW_(NH3)FC_(NH3).

Referring again to FIG. 2, the controller 36 controls operation of theinjector 62 based in-part on the chemical model 82 and a desiredreductant storage setpoint to determine an amount of reductant 66 to beinjected, as described herein. The controller 36 may determine a longterm factor (i.e., a long term correction coefficient) corresponding tothe reductant storage based on monitoring the one or more sensors 54,56, and may more precisely control the amount of injected reductantprovided by the injector 62. For example, the controller 36 determines areductant injector energizing time, long term, correction coefficient tofurther reduce or eliminate discrepancy between the chemical model 82and actual NO_(x) emissions at the outlet of the SCR device 52.Alternatively, or in addition, the controller 36 determines a reductantset-point correction (i.e., a short term correction factor) to reduce oreliminate discrepancy between the chemical model 82 and actual NO_(x)emissions at the outlet of the SCR device 52. That is, the chemicalmodel 82 may be updatable by the short term correction factor, and thelong term factor taken from a lookup table 100 (see FIG. 4) may beapplied directly to the DEF injector control (i.e., controller 36).Accordingly, the supply of reductant 66 may be utilized moreefficiently. The controller 36 may control the amount of reductantsupplied to the SCR device 52 to regulate the reductant storage level(i.e., amount stored by the substrate 68).

In one or more examples, the percentage of NOx that is removed from theexhaust gas 44 entering the SCR device 52 may be referred to as aconversion efficiency of the SCR device 52. The controller 36 maydetermine the conversion efficiency of the SCR device 52 based on theNOx_(in) and NOx_(out) signals 74, 76 generated by the respective NOxsensors 54, 56. For example, the controller 36 may determine theconversion efficiency of the SCR device 52 based on the followingequation:

SCR_(eff)=(NOx_(in)−NOx_(out))/NOx_(in)   (7)

Reductant (e.g., NH₃) slip may also be caused because of an increase inthe temperature of the SCR catalyst. For example, NH₃ may desorb fromthe SCR catalyst of the substrate 68 when the temperature increases attimes when the NH₃ storage level is near to the maximum NH₃ storagelevel. NH₃ slip may also occur due to an error (e.g., storage levelestimation error) or faulty component (e.g., faulty injector) of theemissions control system 34.

Typically, the controller 36 estimates an NH₃ storage level of the SCRdevice 52 based on the chemical model 82. In one or more examples, theNH₃ storage set-point (“set-point”) is capable of being calibrated. Thatis, the NH₃ storage set-point may be a function of exhaust flow rate andtemperature. Based on the current exhaust flow and temperature, theset-point may be defined.

The controller 36 uses the chemical model 82 to estimate the currentstorage level of NH₃ in the SCR device 52, and the storage levelgovernor provides feedback to the injection controls to determine theinjection rate of reductant to provide NH₃ for reactions according tothe chemical model 82, and to maintain a target storage level. Theset-point may indicate a target storage level for given operatingconditions (e.g., a temperature of the SCR catalyst of the substrate68). Accordingly, the set-point may indicate a storage level (S) and atemperature (T) of the SCR device 52, see FIG. 3. The set-point may hedenoted as (S, T). The controller 36 controls the reductant injector 62to manage the amount of reductant injected into the exhaust gas 44 toadjust the storage level of the SCR device 52 to the set-point. Forexample, the controller 36 commands the injector 62 to increase ordecrease the storage level to reach the set-point when a new set-pointis determined. Additionally, the controller 36 commands the reductantinjector 62 to increase or decrease the storage level to maintain theset-point when the set-point has been reached.

The controller 36 may use the chemical model 82 of the SCR catalyst ofthe substrate 68 to predict the NO_(x) concentration in the exhaustgases 44 entering the SCR device 52. Further, based on the predictedNO_(x) concentration, the controller 36 may determine an amount of NH₃needed to dose the exhaust gases 44 to satisfy the emissions threshold.The controller 36 may implement an adaptive semi-closed loop controlstrategy to maintain the performance of the SCR device 52 according tothe chemical model 82, where the controller continuously learns one ormore parameters associated with the chemical model 82 according to theongoing performance of the motor vehicle 20.

In one or more examples, the predetermined value may be determined basedon a specified statistic such as a standard deviation, for example 1.5standard deviations. Further, the predetermined value may be calibratedto a modeled downstream NO_(x) value. The measured downstream NO_(x) isthus normalized against the expected error of the downstream NO_(x)sensor 56. The normalized error, 1.5 in this example, may then becompared to the threshold for entry into steady state slip detectionlogic. The predetermined value of the concentration of the NO_(x) thatis used as the threshold for comparison, in such cases, is computedbased on the earlier values of the NO_(x) measured by the NO_(x) sensor56. In other words, in the above scenario, the 37.5 ppm is used as thethreshold value because 37.5 is the 1.5 standard deviation value ofearlier NO_(x) measurements. It should be noted that in one or moreexamples, the NO_(x) measurement and predicted value used may be aNO_(x) flow rate, or any other NO_(x) attribute (i.e., instead of theNO_(x) concentration).

A dosing governor (not shown) may be controlled by the controller 36,and is configured to monitor the reductant storage level (i.e., in thesubstrate 68 of the SCR device 52) generally predicted by the SCRchemical model 82, and compares the predicted reductant storage level toa preprogrammed, desired, reductant storage level. Deviations betweenthe predicted reductant storage level and the desired reductant storagelevel may be continuously monitored, and a dosing adaptation (i.e., boththe short term correction factor and the long term factor) may betriggered to increase or decrease reductant dosing in order to eliminateor reduce the deviation.

For example, the reductant dosing rate may be adapted to achieve adesired NO_(x) concentration or flow rate in the exhaust gas 44downstream of the SCR device 52, or achieve a desired NO_(x) conversionrate. A desired conversion rate may be determined by many factors, suchas the characteristics of SCR catalyst type and/or operating conditionsof the ICE system 30 (e.g., engine 32 operating parameters). To achievean optimal reductant dosing rate, the short term correction factor maybe applied to the SCR chemical model 82 that generally represents themodeled NH3 storage. If the modeled and requested storage differ, thendosing is modified to achieve the desired storage. That is, the longterm factor may be applied directly to the injector energizing time, andmay increase or decrease dosing accordingly. The short term correctionmay be applied instantaneously, but the long term correction is appliedonly after a period of time.

Over time, inaccuracies of the SCR chemical model 82 may compound toappreciative errors between modeled SCR reductant storage level andactual storage level. Accordingly, the SCR chemical model 82 may becontinuously corrected to minimize or eliminate errors. One method forcorrecting the SCR chemical model 82 includes comparing the modeled SCRdischarge exhaust gas NO_(x) levels to the actual NO_(x) levels,measured by the downstream NO_(x) sensor 56, to determine a discrepancy,and subsequently correcting the SCR chemical model 82 to eliminate orreduce the discrepancy. Because the downstream NO_(x) sensor 56 may becross-sensitive to the reductant 66 and the exhaust gas NO_(x), it iscritical to distinguish between reductant measurements and NO_(x)measurements as reductant slip may otherwise be confused withinsufficient NO_(x) conversion.

A passive analysis technique may be used to distinguish betweenreductant measurements and NO_(x) measurements, is a correlation methodthat includes comparing the upstream NO_(x) concentration, measured bythe upstream NO_(x) sensor 54, to the downstream NO_(x) concentration,measured by the downstream NO_(x) sensor 56. If the difference inconcentration shows a diverging trend (i.e., an increasing difference),this may indicate an increase or decrease in reductant slip. Thecorrelation analysis identifies when the measurements from thedownstream NO_(x) sensor 56 is following the pattern of measurementsfrom the upstream NO_(x) sensor 54 (i.e., the two sensor measurementsare moving alike). The correlation is a statistical measure of thestrength and direction of a linear relationship between the two NO_(x)sensors 54, 56.

For example, the comparison includes a correlation method which includescomparing the downstream NO_(x) concentration with the upstream NO_(x)measurements, or the predicted NO_(x) measurements, wherein divergingconcentration directions can indicate an increase or decrease inreductant slip. For example, if the upstream NO_(x) concentrationdecreases and downstream NO_(x) concentration increases, reductant slipcan be identified as increasing. Similarly, if the upstream NO_(x)concentration increases and downstream NO_(x) concentration decreases,reductant slip can be identified as decreasing. Thus, the divergencebetween the two sequences of NO_(x) measurements can be used todetermine a dosing status of the SCR device 52.

Alternatively, or in addition, the comparison may include a frequencyanalysis. NO_(x) signals 74, 76 generated by the NO_(x) sensors 54, 56may include multiple frequency components (e.g., high frequency and lowfrequency) due to the variation of the NO_(x) and reductantconcentrations during the modulation/demodulation. High frequencysignals generally relate only to NO_(x) concentration, while lowfrequency signals generally relate to both NO_(x) concentration andreductant concentration. High frequency signals for upstream NO_(x) anddownstream NO_(x) are isolated and used to calculate a SCR NO_(x)conversion ratio, which is then applied to the isolated, low-pass,upstream NO_(x) measurement to determine a low frequency downstreamNO_(x) measurement. The calculated low frequency downstream NO_(x)measurement is then compared to the actual isolated low frequencydownstream NO_(x) measurement, wherein a deviation between the twovalues may indicate reductant slip.

A drawback of passive analysis techniques (i.e., short term techniques),such as the correlation method and frequency method described above, isthe reliance on proper operation of two NO_(x) sensors 54, 56. Forexample, a faulty upstream NO_(x) sensor 54 may generate a NO_(x) signal74 that is lower than the actual NO_(x) level proximate the upstreamNO_(x) sensor causing the SCR chemical model 82 to predict higherreductant storage levels than the actual storage level. Accordingly,NO_(x) breakthrough would be incorrectly identified as reductant slip,and reductant dosing would be commanded such that NO_(x) breakthroughwould be exacerbated (i.e., reductant dosing would be decreased).Further, the SCR chemical model 82 would be updated using the inaccurateupstream NO_(x) measurement, and the exacerbated NO_(x) breakthroughwould endure. Additionally or alternatively, in a similar manner, areductant slip may be incorrectly interpreted as NO_(x) breakthrough.

In general, the passive analysis, or short term, techniques may be usedto partially predict the presence of NH3 slip and/or NOx breakthrough.However, merely applying the short term techniques will not compensatefor system drift, part-to-part variations, and other factors, thuscausing the wrong slip decisions to be made, leading to wrong short termstorage level corrections. Wrong short term storage level correctionsmay lead to the wrong long term adaptation decisions. That is, if asystem drift issue is present, merely applying any short term techniquemay cause saturation of the NH3 slip and/or NO_(x) breakthroughpredictions. Therefore, the emissions control system 40 applies both ashort term correction and a long term correction. More specifically, along term adaptation that is only dependent on accumulated error isapplied.

Referring to FIG. 4, a map or lookup table 100 may be stored in theelectronic storage medium 72 of the controller 36 (see FIG. 1) for useby the processor 70 when executing a Long Term Adaptive (LTA) control aspart of the SCR assembly 48 of the emissions control system 34. Thetable 100 may include multiple rows of integrated short term normalizederror categories, or values, 102 that include both NH3 slip error rowsand NO_(x) breakthrough error rows. That is, the normalized errorrelative to NH3 slip includes multiple values 102S (i.e., rows)expressed as positive values, and the normalized error relative toNO_(x) breakthrough includes multiple values 102B (i.e., rows). Themultitude of columns in the lookup table 100 are associated with currentlong term adaptation factors 104. In operation, the processor 70 of thecontroller 36 cross references the current long term adaptation factor104 to the integrated short term normalized error 102 to determine a newlong term adaptation factor 106.

Referring to FIG. 5, a flowchart of a method 200 for adaptive SCRcontrol and Long Term Adaptive (LTA) entry is illustrated. The method200 may be implemented by the controller 36 and/or one or more electriccircuits. The method 200 may be implemented by execution of logic thatmay be provided or stored in the form of computer readable and/orexecutable instructions in the storage medium 72 of the controller 36.At block 202, at least one short term enablement criteria is met.Examples of short term enablement criteria include: a normalizedchemical model error is greater than a first threshold, a NO_(x)gradient is less than a second threshold, a reductant (e.g., NH3)consumed is greater than a third threshold (i.e., SCR device stability),a temperature window (i.e., a temperature being greater than a fourththreshold and less than a fifth threshold), a temperature gradient beingless than less than a sixth threshold, a reductant storage leveldeviation being less than a seventh threshold, and a combustion mode.

The normalized chemical model error being greater than the firstthreshold, and as part of the short term enablement, may be generallythe difference between the model predicted NO_(x) from SCR chemicalmodel 82 and the actual NO_(x) normalized by magnitude. The NO_(x)gradient being less than a second threshold, and as part of the shortterm enablement, may be a rate of change (e.g., ppm/s) of NO_(x)entering the SCR device 52. A large gradient may be an indicator of ahighly transient vehicle maneuver where corrections are desirable. Thereductant (e.g., NH3) consumed being greater than a pre-establishedthreshold is generally a stability criteria for the SCR device 52.

The temperature window, as part of the short term enablement, isgenerally indicative of a temperature being higher than a lowtemperature threshold and less than a high temperature threshold. Thetemperature window criteria may allow alignment of short term correctionwith the operating range where the SCR chemical model 82 is mostaccurate. That is, the SCR chemical model 82 may not be as accurate atvery low temperatures where performance is reduced, or at very hightemperatures when there is a propensity of NH3 slip.

The temperature gradient being less than the sixth threshold criteria,and as part of the short term enablement, is indicative of a rate ofchange of temperature at the inlet of the SCR device 52. A largegradient may be an indicator of a highly transient vehicle maneuverwhere exhaust correction may be undesirable.

The reductant storage level deviation being less than a sevenththreshold criteria, and as part of the short term enablement, allowsalignment of short term correction with the operating range where theSCR chemical model 82 is most accurate. The SCR chemical model 82 maynot be as accurate when the actual reductant storage level on the SCRcatalyst is much higher or lower than the setpoint (i.e., sevenththreshold).

The combustion mode criteria allows blocking of short term correction oncombustion mode (e.g., DPF regeneration, SCR warmup, and others).Certain combustion modes may have a higher propensity for increasedtemperatures, decreased storage levels, and increased NH3 slip. Undersuch modes, or conditions, short term corrections should be avoided.

At block 204, and if the short term enablement criteria is met, thecontroller may calculate the normalized chemical model error. Inputs forthe normalized chemical model error include the signals 74, 76 from therespective upstream and downstream NO_(x) sensors 54, 56, and the SCRchemical model 82. The SCR chemical model 82 may be formed and developedfrom inputs associated with the temperature sensor 58, the NO_(x)sensors 54, 56, and the previous SCR chemical model 82 contained withinthe controller 36. The normalized chemical model error may equal thedelta between the measured NO_(x) associated with the downstream NO_(x)signal 76 and the modeled, or predicted, downstream NO_(x) that isnormalized based on magnitude of values.

At block 206, the normalized chemical model error, associated with shortterm control, is integrated. In one example, the task rate for thisintegration may be about fifty milliseconds. At block 208, if theintegrated normalized chemical model error exceeds a threshold, themethod 200 may proceed toward a long term adaptive factor determination.At block 210 and proceeding with the long term adaptive factordetermination, a determination of whether long term adaptationenablement criteria is met. The long term adaptation enablementcriteria(s) may be similar to the short term adaptation enablementcriteria(s) except that the respective thresholds may be different. Thatis, the thresholds between the long term adaptation enablementcriteria(s) may be independent from the short term adaptation enablementcriteria(s). Generally, all thresholds may be SCR strategy and hardwaredependent.

Examples of long term enablement criteria may include: the normalizederror is greater than an eighth threshold, the NO_(x) gradient is lessthan a ninth threshold, the reductant (e.g., NH3) consumed is greaterthan a tenth threshold (i.e., SCR device stability), the temperature isgreater than an eleventh threshold and less than a twelfth threshold, atemperature gradient is less than a thirteenth threshold, a reductantstorage deviation is less than a setpoint (i.e., fourteenth threshold),and the combustion mode.

In one embodiment, the normalized error threshold for the long termadaptation (i.e., eighth threshold) may be substantially larger than thenormalized error threshold for the short term correction (i.e., firstthreshold). Furthermore, the various thresholds of the long termadaptation enablement criteria may be about twice as large as therespective thresholds of the short term adaptation criteria. In otherembodiments, the thresholds may be about equivalent.

At block 212, and if the long term adaptation enablement criteria ismet, the integrated normalized chemical model error 102 and the currentlong term adaptive factor 104 are utilized as inputs for the map orlookup table 100 to determine a new (i.e., subsequent) long termadaptive factor 106. For example, if the integrated short termnormalized error 102 is 0.3 (i.e., an integrated short term normalizederror associated with NH3 slip) and the current long term adaptationfactor is about 0.8, the new long term adaptation factor 106 would beabout 0.77 (i.e., as an example portrayal). The new long term adaptationfactor 106 may then be used to compensate for system drift and/orpart-to-part variation. In, one example, the new long term adaptationfactor 106 may generally multiply the DEF injector energizing time(i.e., the amount of time the injector remains open).

The technical solutions described herein facilitate improvements toemissions control systems used in combustion engines, such as those usedin vehicles. For example, the technical solutions determine storagecorrection and adaptation based on integration of a smaller error thanwhat is used to enter a steady state reductant slip detection logic, theerror indicative of a difference between downstream NO_(x) sensormeasurement and downstream NO_(x) model. Such improvements facilitateprevention of cycling of steady state reductant slip detection when theNO_(x) error is just high enough to cause a steady state reductant slipdetection event, but the error is low enough to cause the system tocycle in and out of the steady state reductant slip detection withoutany adapts.

Further advantages and benefits include a system 40 configured to treatshort term and long term adaptation independently. This independencecontributes toward improved adaptation robustness, decreased falsefailures, and a reduction in the potential for DEF crystallization.

While the above disclosure has been described with reference toexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from its scope. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the disclosure without departing from the essentialscope thereof. Therefore, it is intended that the present disclosure notbe limited to the particular embodiments disclosed, but will include allembodiments falling within the scope thereof.

What is claimed is:
 1. An emissions control system for treating exhaustgas of a combustion engine, the emissions control system comprising: aSelective Catalytic Reduction (SCR) device adapted to reduce emissions;a reductant injector adapted to inject a reductant into the SCR device;a downstream NO_(x) sensor disposed downstream of the SCR device; acontroller including a processor and an electronic storage medium; aniterative model stored in the electronic storage medium; and a lookuptable stored in the electronic storage medium, and wherein the processoris configured to perform short term and long term adaptive control by:confirming at least one short term enablement criteria is met;calculating a normalized chemical model error utilizing in-part theiterative model and a downstream NO_(x) signal received from thedownstream NO_(x) sensor; integrating the normalized chemical modelerror to produce an integrated normalized chemical model error;confirming that the integrated normalized chemical model error exceedsan error threshold; proceeding toward the long term adaptive control;confirming at least one long term adaptation enablement criteria is met;applying a current long term adaptive factor and the integratednormalized chemical model error to the lookup table to determine a newlong term adaptive factor; and multiplying the new long term adaptivefactor against an energization time of the reductant injector.
 2. Theemissions control system set forth in claim 1, further comprising: anupstream NO_(x) sensor disposed upstream of the reductant injector andthe SCR device, wherein the processor is configured to receive anupstream NO_(x) signal from the upstream NO_(x) sensor to calculate thenormalized chemical model error.
 3. The emissions control system setforth in claim 1, wherein the normalized chemical model error isassociated with the difference between a model-predicted NO_(x) leveltaken from the iterative model, and an actual NO_(x) level taken formthe downstream NO_(x) signal.
 4. The emissions control system set forthin claim 3, wherein the normalized chemical model error is normalized bymagnitude.
 5. The emissions control system set forth in claim 1, whereinthe at least one short term enablement criteria includes at least one ofa normalized error being greater than a first threshold, a NO_(x)gradient being less than a second threshold, a reductant-consumed beinggreater than a third threshold, a temperature being greater than afourth threshold and less than a fifth threshold, a temperature gradientbeing less than a sixth threshold, a reductant storage level deviationbeing less than a seventh threshold, and a combustion mode.
 6. Theemissions control system set forth in claim 5, wherein the at least onelong term enablement criteria includes at least one of the normalizederror being greater than an eighth threshold, the NO_(x) gradient beingless than a ninth threshold, the reductant consumed is greater than atenth threshold, the temperature is greater than an eleventh thresholdand less than a twelfth threshold, the temperature gradient is less thana thirteenth threshold, the reductant storage deviation is less than afourteenth threshold, and the combustion mode.
 7. The emissions controlsystem set forth in claim 1, wherein the at least one short termenablement criteria is independent from the at least one long termenablement criteria.
 8. An emissions control system for treating exhaustgas of a combustion engine, the emissions control system comprising: aSelective Catalytic Reduction (SCR) device; a first NO_(x) sensor; and acontroller configured to perform short term and long term adaptivecontrol by: comparing a first NO_(x) measurement from the first NO_(x)sensor with a predicted NO_(x) value based at least in-part on aninitial chemical model, and in response to a short term enablementcriteria being met: calculating a normalized chemical model error;integrating the normalized chemical model error; and calculating a newlong term adaptive factor if the integrated normalized chemical modelerror exceeds a threshold.
 9. The emissions control system set forth inclaim 8, further comprising: a lookup table stored in the controller andconfigured to cross reference a current long term adaptive factor to theintegrated normalized chemical model error to calculate the new longterm adaptive factor.
 10. The emissions control system set forth inclaim 9, wherein the normalized chemical model error is equal to a deltabetween the first NO_(x) measurement and a predicted NO_(x) value basedon the initial chemical model, and normalized based on magnitude. 11.The emissions control system set forth in claim 10, wherein the firstNO_(x) sensor is located downstream from the SCR device.
 12. Theemissions control system set forth in claim 8, further comprising: asecond NO_(x) sensor, wherein the normalized chemical model error isbased on the initial chemical model, the first NO_(x) measurement, andan upstream NO_(x) measurement from the second NO_(x) sensor locatedupstream from the SCR device, and wherein the first NO_(x) sensor islocated downstream from the SCR device.
 13. The emissions control systemset forth in claim 12, further comprising: a temperature sensorconfigured to send a temperature measurement to the controller, whereinthe initial chemical model is generated by the controller and is atleast based on the temperature measurement, the upstream NO_(x)measurement, and the first NO_(x) measurement.
 14. The emission controlsystem set forth in claim 9, wherein the enablement criteria is a shortterm enablement criteria.
 15. The emission control system set forth inclaim 14, wherein the short term enablement criteria includes at leastone of a normalized error being greater than a first threshold, a NO_(x)gradient being less than a second threshold, a reductant-consumed beinggreater than a third threshold, a temperature being greater than afourth threshold and less than a fifth threshold, a temperature gradientbeing less than a sixth threshold, a reductant storage level deviationbeing less than a seventh threshold, and a combustion mode.
 16. Theemission control system set forth in claim 15, wherein the long termadaptive factor determination is conducted when a long term adaptationenablement criteria is met.
 17. The emission control system set forth inclaim 16, wherein the long term adaptation enablement criteria isindependent from the short term enablement criteria.
 18. The emissioncontrol system set forth in claim 16, wherein the long term adaptationenablement criteria includes at least one of the normalized error beinggreater than an eighth threshold, the NO_(x) gradient being less than aninth threshold, the reductant consumed is greater than a tenththreshold, the temperature is greater than an eleventh threshold andless than a twelfth threshold, the temperature gradient is less than athirteenth threshold, the reductant storage deviation is less than afourteenth threshold, and the combustion mode.
 19. The emission controlsystem set forth in claim 18, wherein the eighth threshold is greaterthan the first threshold.
 20. The emission control system set forth inclaim 8, further comprising: a reductant injector, wherein the new longterm adaptive factor is generally multiplied by an energization time ofthe reductant injector.